1. Field of the Invention
This invention relates to an air conditioning system, and more particularly to an air conditioning system for an automatic vehicle equipped with a clutchless compressor to which a driving force is constantly transmitted from an engine installed on the vehicle.
2. Description of the Prior Art
Conventionally, an air conditioning system of this kind includes a clutchless compressor to which a driving force is constantly transmitted from an engine via a pulley, a condenser for causing a gaseous refrigerant compressed to high temperature and high pressure by the clutchless compressor to change to a high-pressure liquid refrigerant through heat exchange with external air, an expansion valve for adiabatically expanding the liquid refrigerant to thereby change the refrigerant into a low-pressure atomized refrigerant, and an evaporator for evaporating the atomized refrigerant through heat exchange with the external air.
As the above clutchless compressor, there has been proposed a variable capacity swash plate compressor in which the inclination angle of a swash plate thereof changes according to a suction pressure to thereby change a length of piston stroke for increasing or decreasing the delivery quantity of the compressor.
When a variable capacity swash plate compressor whose minimum delivery quantity is not reduced to zero is adopted as the clutchless compressor, if thermal load on the compressor is low (which corresponds to a state in which the clutch of a compressor of a clutch-equipped type is disengaged), the evaporator is cooled by the refrigerant to cause a surface of the evaporator to be frosted to freeze the evaporator, which makes it difficult to ventilate the evaporator, and in some cases results in a spoiled refrigerating function of the compressor.
To eliminate the above inconvenience, a technique of reducing the minimum delivery quantity to zero when thermal load is decreased has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 7-293437. This technique is based on the fact that a variable capacity swash plate compressor has a structure which permits the inclination angle of the swash plate to be reduced to zero (state in which the swash plate is at right angles to a drive shaft of the compressor), whereby the minimum delivery quantity can be reduced to zero.
Another technique for the same purpose has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 7-290940, in which a shortcut passage bypassing the evaporator is provided for the freezing circuit to directly connect the liquid tank and the compressor via the shortcut passage when thermal load on the compressor is low, to thereby inhibit refrigerant from circulating through the evaporator.
However, in the case of the technique of reducing the minimum delivery quantity to zero, the compressor is required to be constructed such that the refrigerant can be circulated within the compressor and the delivery quantity can be recovered from zero (by increasing the inclination angle of the swash plate from zero). This is because if the inclination angle of the swash plate is equal to zero when the compressor is started, it is impossible to obtain the back pressure of pistons.
Further, since the amount of refrigerant circulating through the compressor is largely reduced to zero or almost zero, it is required to eliminate heat generated in the compressor or cope with the shortage of lubricant which is to be supplied from the circulating refrigerant.
On the other hand, in the case of the technique of providing the shortcut passage for the freezing circuit, which bypasses the evaporator for use when the thermal load is low, the liquid tank is short-circuited or directly connected to the compressor to return the liquid refrigerant to the compressor, which causes so-called liquid compression, resulting in breakage of the compressor.